The present invention relates to a method of manufacturing a multiple mirror reflector for a land based telescope for the reflection of short-wave electromagnetic radiation which comprises bonding a reflective layer to a concave surface of a substrate and mounting the substrate on a dished rigid support.
It is known that an increase in the detail of information collected from very faint objects in space may be obtained by an increase in the diameter size of the telescope reflectors used by astronomers. Telescopes with reflectors larger than the known large reflector of the 5 meter telescope on Palomar Mountain are necessary to improve the collection of data from short wave radiations. Large reflectors of several meters in diameter for collecting relatively long radio waves from space are known and used successfully because the difficult engineering problems and engineering tolerances required to make such reflectors are less stringent than the tolerances required for making reflectors which operate in the short wave electromagnetic radiations range, including visible and/or infra-red light radiations. In general, for a primary collecting reflector to be effective in collecting information from short wave radiations from faint and distant objects the reflector has to have both a large electromagnetic radiation collecting area, as represented by the diameter of a single dish reflector or the diameter of each of several dish reflectors, and an accurate surface profile which varies across the diameter of each dish reflector by preferably, no more than about one tenth of a wave length of the radiation received. In the known large reflectors which collect the relatively long radio waves having wave lengths, of a few centimeters, for example, a surface profile deviation of the reflector used of approximately 1 millimeter are attainable by normal high precision engineering practice. For shorter wavelength radiations, in the visible light and/or infra-red regions, a surface profile deviation of the reflector of approximately a few millionths of a centimeter is required. The engineering problems for manufacturing reflectors consequently become more acute with such an increase in surface accuracy in relation to the diameter of the reflector, and in relation to a reduction in wavelength of the radiations under examination. Various attempts to improve the efficiency of radiation collection of short wave length radiations have been made as for example, in the known large telescopes which have a single 5.1 meter (200 inch) diameter reflector and in a Russian 6 meter diameter reflector. Various other attempts to improve the efficiency of collecting short wave radiations include
(a) a multiple mirror type telescope which comprises an array of several medium sized telescopes accommodated on the same altazimuth mounting, PA0 (b) a primary mirror type telescope where the mirror comprises a section of a sphere composed of closely fitting hexagonal mirrors which can rotate about a vertical axis, PA0 (c) a primary mirror type telescope where a fully steerable dish is composed of a large number of separate reflective elements which are individually focussed and aligned, and PA0 (d) a conventional type telescope where the primary reflector mirror comprises a thin monolithic primary mirror of low overall weight.
A considerable saving in cost is effected in the later case by reducing the overall weight of the primary mirror and in one example a primary reflector mirror of this type (type d) has been manufactured with a reflector of 3.8 meter in diameter.
However, in order to obtain the necessary wide range of data collection for making readings in optical spectroscopy and to determine for example, the distance, composition, luminosity, the red shift, the rotation speed and the temperature of distant objects a collecting area of the primary reflector mirror or mirrors should be even larger and preferably the reflector should at least be equivalent to a 25 meter diameter mirror.
A primary mirror telescope which has a fully steerable dish reflector composed of a number of separate reflective, individually focussed elements is known for reflecting electromagnetic radiations in the millimeter and submillimeter wave bands from United Kingdom Pat. No. 1,546,645. To produce the separate reflective elements there is described in the United Kingdom Pat. No. 1,546,645 a method of producing by moulding a plurality of substantially identical reflectors to give at least one series of the reflectors. These reflectors are described as being capable of being used individually or assembled together to form a large reflector. The method of producing the reflector, or series of reflectors, however is by moulding. In the known method, each reflector comprises a concave reflector foil bonded to a support member and is formed from a resiliently-flexible plastics foil which is coated on at least one surface with a radiation reflective layer, the foil is applied to the surface of a master moulding tool and tension is applied to the foil so that, at least at its periphery, stretching occurs and the foil takes up the profile of the convex surface, the foil being bonded to the support member while positioning the concave surface of the support member in close proximity to, but substantially uniformly spaced from, the tensioned foil, filling the space between the support member and the foil with an adhesive liquid resin, causing the liquid resin to set whilst maintaining the foil in the stretched condition and separating the master moulding tool from the bonded foil and support member.
The reflector elements of United Kingdom Pat. No. 1,546,645 have the disadvantage in that the reflective layer of each element is formed from the tensioned concave reflector foil which is stretched, at least at its periphery, in order to take up the profile of the convex surface of the master moulding tool. Consequently the radiation reflective layer comprising the resiliently flexible plastics foil of each element, is not of uniform thickness over the whole of its surface. Moreover, as the radiation reflective layer is very thin, as commonly is the case for vapour deposited metal coated plastics foil, the stretched radiation reflective layer may also become radiation transparent to short wave electromagnetic radiations. Thus, with stretching, both the radiation reflective layer and the plastics foil over the convex surface of the mould result in a differential thickness change which alters the resultant reflective profile of the reflector and which, because the wavelengths of short wave, infra-red and optical light, are smaller than the wavelengths of radio waves by a factor of approximately 100,000 causes a surface change in the profile of the stretched plastics foil which is greater than the aforesaid tolerance limits of a few millionths of a centimeter.
In another known method in the manufacture of a primary mirror reflector of a close fitting segmented mirror construction, a similar disadvantage of surface profile change which is outside the tolerance limits has been found to occur. For example, in the construction of a close fitting segmented mirror reflector the primary mirror may be composed of more than 1000 separate reflective elements which are assembled together and are individually focussed and aligned on a continual basis dependent upon the primary reflector's profile size, shape and optical focussing requirements. Each reflector element, furthermore, during its manufacture has to be separately cast, ground and polished to an appropriate correct profile which is then shaped in surface area to ensure a close fitting segmented construction. The reflector element preferably is manufactured to a general profile shape by moulding a material, such as, glass, particularly a low thermal coefficient of expansion glass, which is subsequently subjected to grinding, polishing and shaping processes. The shaped reflector element is then stress relieved so as to form a geometrically stable profile or part profile for accurate assembly and alignment on the segmented reflector. This method is clearly a long and expensive process particularly as each reflector element of any one series of elements has slightly different physical properties so thay they respond differentially to any change in the environmental conditions, such as a change in temperature.
Deformation of the profile of such reflector elements has been found to be unpredictable and uncontrollable even when the reflector elements have been prepared with great care in a desired constant and standard manner. Material, for example may be required to be removed from the edge of the profiled element, for the final purpose of removing any edge run-off inaccuracies, or for forming a shaped hexagonal element of one series of reflective profiled elements from another slightly larger series of shaped elements; and, any removal of material from one profiled element results in producing an element which differs from other elements in profile changes in use.